A conventional washing machine includes a rotary drum, having a substantially horizontal or slanted rotational axis, for accommodating laundry therein; a water tub that incorporates the rotary drum therein and is supported in a washing machine main body; a supporting metallic part for supporting the water tub; a washing machine base for supporting the washing machine main body; a motor for rotating the rotary drum; an input setup unit for setting up operations of the washing machine; and a controller for controlling a washing operation of the washing machine set up by the input setup unit and controlling the motor. Under the control of the controller, washing, rinsing and water-extracting processes of the washing machine are regulated precisely.
When the washing and rinsing processes are completed, laundry in the rotary drum contains water therein, so the water-extracting process is performed to remove water from the laundry by way of rotating the rotary drum. At this time, however, the laundry articles may be placed in an imbalanced state within the rotary drum with regard to the rotation movement during the water-extracting process depending on the types, materials and shapes thereof. In such a case, the rotary drum and the like would vibrate considerably, thereby making noise.
Thus, in order to detect an abnormal vibration during the water-extracting process, it has been proposed to accommodate the laundry and detergent in a rotary drum supported rotatably in an inner frame, the inner frame being in turn supported in an outer frame by a buffering structure such as a spring, and to detect a mechanical vibration of the inner frame by means of a vibration detecting unit disposed in the outer frame (see, for example, Japanese Patent Laid-open Application No. S61-98286: Reference 1).
In this method, the water-extracting process is performed by executing first a balancing operation of rotating the rotary drum with a motor driven at a low speed so that the laundry accommodated in the rotary drum is uniformly attached to the inner wall of the rotary drum by a centrifugal force, and then rotating the rotary drum at a higher rotational speed. If an abnormal vibration occurs during these steps, the rotary drum is immediately stopped.
Further, there have been proposed other methods for detecting an abnormal vibration due to an imbalanced distribution of laundry in a rotary drum before rotating the rotary drum at a high rotational speed, to thereby enable an execution of a safe and high-efficiency water-extracting process. For example, Japanese Patent Laid-Open Application No. H6-170080 (Reference 2) discloses a method for detecting an abnormal vibration of a washing machine that includes an induction motor for rotating a rotary drum and an inverter circuit for driving the induction motor. In the method, a washing operation where the rotary drum is rotated in forward and backward directions, a balancing operation where the rotary drum is rotated at a low rotational speed, and a water-extracting operation where the rotary drum is rotated at a high rotational speed are successively performed in the order. Upon starting the balancing operation, an effective current is detected from an output of the inverter circuit, and a difference of current is calculated between the maximum value and the minimum value of the effective current. Then, the calculated difference of current is compared with a preset threshold current value representing an excessive vibration. If the difference of current exceeds the preset threshold, an excessive vibration is detected based on the current, and a warning of an occurrence of excessive vibration is outputted.
However, in case of the configuration disclosed in Reference 1, during the balancing operation wherein the rotary drum is rotated at a low rotational speed, an abnormal vibration may not yet be detected due to the small amplitude of a mechanical vibration, even if there is an imbalanced distribution of laundry articles within the rotary drum. Since the amplitude of the vibration does not become large enough to be detected until the rotational speed of the rotary drum is increased to a high rotational speed, it may be difficult to detect the occurrence of the abnormal vibration before rotating the rotary drum at the high rotational speed. Accordingly, the rotary drum can be stopped only after the abnormal vibration has already occurred. Therefore, there is a high risk that the laundry or the washing machine may be subject to a damage, and an unnecessarily greater amount of time may be required until the rotary drum is stopped.
Further, the method of Reference 2, which detects an abnormal vibration indirectly from an effective current of the induction motor, is based on the assumption that an imbalance of laundry is reflected on the effective current of the induction motor and that the imbalanced state leads to an abnormal vibration. However, a variation of the effective current of the induction motor can be caused not only by an imbalanced distribution of laundry within the rotary drum but also by mechanical factors, e.g., due to a bearing of the induction motor or the like. Further, since an occurrence of an excessive vibration is determined by comparing the variation in the effective current with a preset threshold current value, excessive vibration warnings may be issued more often than necessary, thereby stopping the rotary drum too frequently.